1. Field of the Invention
This invention relates to an ink film cassette holding an ink film, and a reel to be used for thermal transfer recording device.
2. Description of the Related Art
The thermal transfer printer is provided with a platen roller and a thermal head freely pressed against and separated from the platen roller, and conveys a recording paper through the contacting surface between the platen roller and the thermal head. An ink film which has an ink with thermally fusible or sublimating properties applied to one surface is conveyed into the contacting surface between the recording paper and the thermal head. This ink film is unwound from a reel on the supply side and rewound on a reel on the take-up side. When a color image is reproduced by one thermal head on the recording paper, the printer uses the ink film which has the inks in the colors of yellow, magenta, and cyan applied sequentially in the order on the surface of a thin film base.
In recent years, the thermal transfer printer is applied to an ink film cassette which holds a supply reel and a rewind reel inside and is freely attached to and detached from the main body for improving the efficiency of the setting work of the ink film. The ink film cassette is a consumable product and is replaced with a new supply after the ink film held therein has been used up.
The ink film, during the printing process, is drawn out of the supply reel and is then conveyed in conjunction with the motion of the recording paper by friction force between ink film and the recording paper being conveyed between the platen roller and the thermal head now in a state of being pressed against the platen roller. The ink film paid out of the supply reel is rewound on the rewind reel by the fact that a motor installed in the main body of the thermal transfer printer transmits a rotation to the rewind reel.
The thermal transfer printer is provided in the main body with a supply side torque limiter which is joined to the supply reel and enabled to exert a braking force on the supply reel after the ink film cassette has been set in place. This supply side torque limiter effects continued exertion of a braking force on the supply reel while the ink film is being paid out of the supply reel. As a consequence, the limiter fulfills the purpose of transmitting due tension to the ink film and preventing the ink film from forming wrinkles during the printing process.
Further, inside the main body of the thermal transfer printer, a take-up side torque limiter for limiting driving torque of the rewind reel is disposed between the rewind reel and the motor. This take-up side torque limiter is provided for the following reason besides the reason of transmitting due tension to the ink film during the printing process. The speed of motion of the recording paper and the speed of motion of the ink film must be equalized in the printing unit. Even when the supporting shaft of the rewind reel is rotated at a fixed speed, the speed of rewinding the ink film on the rewind reel is inevitably varied as the roll diameter of the ink film taken up in the rewind reel varies. It is, therefore, provided with the take-up side torque limiter for enabling the speed of rewinding to follow the speed of motion of the ink film in the printing unit and allowing the ink film to be rewound without slacking on the rewind reel.
The torque limiters of the kind that harness the phenomenon of friction have been finding extensive utility to date. The friction type torque limiter is provided with an inner shaft rotatably supported inside a housing, and a friction member held inside the housing to be pressed against the inner shaft. The friction member is to be pressed against the inner shaft and, in consequence of the resultant friction, is enabled to confer a rotational load on the inner shaft and generate a set torque. Grease is applied between the inner shaft and the friction member.
The prior technique incurs the following problems.
In the conventional ink film cassette, the supply reel and the rewind reel are held inside the ink film cassette so as to be smoothly rotated. Namely, the conventional ink film cassette is designed to reduce the slide resistance generated between the supporting shaft of the supply reel and the ink film cassette retaining the supporting shaft, and the slide resistance generated between the supporting shaft of the rewind reel and the ink film cassette retaining the supporting shaft.
No rotational load is transmitted to the supply reel inside the ink film cassette. When the ink film cassette as a consumable product is shipped and is in the process of transportation, therefore, the ink film is loosed under the influence of an external force and paid out readily from the supply reel. As a result, part of the unused ink film is wrinkled and the ink film cassette can no longer be sold.
In some of the various forms of use found for the ink film cassette, an ink film cassette may be possibly removed from the main body of the printer before the ink film held therein has been completely used up, and the partly used ink film cassette may be again set in place in the main body of the printer. When the ink film cassette is removed in a partly used state as described above, the possibility likewise arises that the ink film held therein will be loosed under the influence of an external force and paid out readily from the supply reel and thus the ink film cassette can no longer be sold.
No rotational load is transmitted to the rewind reel inside the ink film cassette. In consequence, when the ink film cassette is removed in a wholly consumed state from the main body of the printer, there ensues under the influence of an external force the possibility that the ink film which is used up and rewound on the rewind reel will be loosed paid out readily from the rewind reel, and suffered to sustain damage.
When the ink film cassette with the loosed ink film is loaded in the main body of the printer, it calls for an extra work of causing the loosed part of the ink film to be rewound on the supply reel or on the rewind reel and entails the problem of complicating the work of loading.
For the purpose of preventing the unused ink film from being paid out of the supply reel or the used ink film from being paid out of the rewind reel, stoppers for inhibiting the supply reel and the rewind reel from rotating must be fixed one each to the supporting shafts of these reels. This fixation of the stoppers turns out to be a very complicated work.
Further, the supply side torque limiter and the take-up side torque limiter are disposed in the main body of the thermal transfer printer. The fact adds to the complexity of the construction of the printer proper and to the cost of the printer proper and, at the same time, prevents the printer proper from being miniaturized.
Since the torque limiters to be disposed inside the main body of the printer are required to possess as high durability as the other components which are provided inside the main body of the printer, specifically the durability that allows normal formation of 60,000 image planes, they must be made of a relatively expensive material which results in increasing the cost of components and the whole cost of the thermal transfer printer.
The ink film is known in numerous kinds which differ in physical properties of film due to differences such as in section thickness of film and friction coefficient of backcoat. In the thermal transfer printer of the construction which has torque limiters disposed on the main body of the printer as described above, it is not easy to alter the set torque. The thermal transfer printer, therefore, must use an ink film cassette of exclusive design and entails the problem of permitting no addition to the kind of film fit for use.
The ink film cassette is also required to offer service of exalted convenience such as by enabling the user to refill it easily.
On the other hand, the torque limiter itself incurs the following problem.
The conventional friction type torque limiter suffers leakage of grease through the boundary between the housing and the inner shaft with an increase in the total number of rotations. When the torque limiter happens to be used where it is exposed to the user's hand, it is at a disadvantage in smearing the users hand with the leaked grease. It possibly entails the disadvantage that the leaked grease will adhere to and smear other components. When the leakage of grease proceeds excessively, there ensues the disadvantage that the friction resistance between the inner shaft and the friction member will grow possibly to the extent of preventing the torque limiter from exactly responding to the set torque.
The conventional friction type torque limiter is not so constructed as to rely on the housing to arrest the inner shaft in the axial direction. In contrast, the position of the inner shaft in the axial direction is solely retained by the intimate contact with the friction member. Particularly when the torque limiter is such that the friction member produces small pressing force and the set torque is small, therefore, there arises the disadvantage that the inner shaft will be pressed down even by feeble force and will be deprived of usefulness.